Interactions of Clq with various IgG systems and with subcomponents Clr and Cls will be examined by physico-chemical methods in an effort to understand molecular events leading to the activation of Cl. Enzymatic studies of Clq and poly d-1 alanylted Clq under conditions where Clq appears to undergo conformational change (pH, solvent, immune complex reaction) will be done to assess conformational change in Clq and to obtain soluble monomeric subunits of Clq and Fc binding Clq peptides> Interactions of Clq, and of materials derived from it, with Ca dependent Ag-Ab systems (antipolypeptide, anti HSA, non-precipitating complexes) and with monomeric and aggregated IgG will be studied by hydrogen exchange and spectroscopy to determine the valence of Clq size and nature of its binding sites, the involvement of conformational change upon reaction, Tyr and Trp involvement in the Clq and Fc binding sites and also characteristics of the IgG-Fc binding site. With a precipitating Ca dependent Ag-Ab system where ligand interaction can be modulated, an attempt to assess the respective roles of ligand induced antibody conformational change and of Fc aggregation on Clq binding will be made. The role of bivalent IgG attachment on Clq binding will be examined. Following kinetic studies of Cl reconstitution from subcomponents, the effects on Clq conformation of interaction with Clr and Cls, alone and in combination, will be examined by hydrogen exchange. By manipulation of calcium dependency of Cl information on intrasubcomponent interaction will be obtained. Calcium exchange studies will be done on Cl to determine dissociation energy, the role and environment of calcium and the stabilization role each subcomponent has in Cl. Calcium exchange will also be performed on calcium induced Cls dimer. Hydrogen exchange on selected subcomponents in reconstituted Cl and Ca exchange will be performed to assess the respective roles of the subcomponents in the activation of Cl and to assess the role of postulated internal rearrangements within the complex which might lead to its activation.